Aircraft are complex and use a vast array of electrical devices such as sensors, weapon systems (for military aircraft) and cockpit displays. Other devices include environmental systems, flight controls, galley equipment, communication gear, weather radar, in-flight entertainment systems, external lighting, etc. Typically, DC power supplies, such as batteries, are insufficient to meet the demands for electricity in operating these devices.
Aircraft are equipped with a number of power generation systems including primary and redundant backup systems to supply power to equipment in an emergency. Primary power is usually provided by AC generators directly connected to the gas turbine engines. Commercial aircraft and many military aircraft are equipped with auxiliary power units (APU), essentially smaller gas turbine engines, which provide an additional power source. The APU supplements the primary power system or replaces it in case of engine failure. If the APU fails, many aircraft carry a ram air turbine (RAT) that can be deployed when needed to provide emergency power to keep critical systems operating long enough to land safely. Some aircraft may also have battery backups.